1. Field of the Invention
The present invention relates to a servo writer and a servo writing method, for writing servo data in a magnetic disk loaded in a magnetic disk drive. More particularly, the invention relates to the servo writer and the servo writing method for writing high-quality servo data in the magnetic disk by optimizing the rotation frequency (rotation speed) of the magnetic disk.
2. Description of the Related Art
Generally, a hard disk drive (HDD), or a magnetic disk drive, is used as a storage device for a desk-top type personal computer or a notebook type portable computer. FIG. 1 is a perspective view showing the internal structure of this magnetic disk drive 1. The magnetic disk drive 1 is a device for recording and reproducing data on a recording medium (disk) 2 by using a magnetic head 3. The recording medium 2 is manufactured by coating and depositing a magnetic recording film on a disk-like substrate. The magnetic disk 2 is rotated at a high speed by a spindle motor 4. The magnetic head 3 is held in a carriage 5. The magnetic head 3 is enabled to move on the magnetic head 2 in its radial direction, by moving the carriage 5 by means of a voice coil motor (VCM) 6.
The magnetic head 3 is thus positioned in a desired place on the magnetic disk 2 by means of the voice coil motor 6. The head 3 is positioned by a so-called embedded servo method. In this method, a plurality of servo data portions are provided in the all track of a whole data face, during manufacturing of a magnetic disk drive, that is, in the process of writing servo data into the magnetic disk 2 (servo writing). These servo data portions are basically composed of cylinder code portions in which cylinder codes are embedded, and correction portions in which data to be used for detecting and correcting positional shifts are written. In the magnetic disk drive 1, a desired track is accessed by the magnetic head 3 by detecting the cylinder codes of the cylinder code portions. The current position of the magnetic head 3 is then detected and corrected by detecting discretely data recorded in the correction portions (several times for each track). The magnetic head 3 can thereby be positioned in a desired position on the magnetic disk 2 with high accuracy.
FIG. 2 illustrates an example of the structure of the spindle motor 4. The components of the spindle motor 4 shown in FIG. 2 are provided under the magnetic disk 2 in the magnetic disk drive 1. A hub 41 is rotated at a high speed together with the magnetic disk 2 attached thereto. A bearing 42 supports the rotary shaft of the hub 41 rotating at a high speed.
A coil 43 is wound around a stator core 44. A control current is supplied to this coil 43 by FPC (Flexible Printed Circuit) 47. A magnet 45 forms an electromagnetic driving element, together with the coil 43. This magnet 45 is connected to the hub 41 via a back yoke 46. A motor base 48 holds the spindle motor 4 having the above-noted structure and is connected to the base of the casing of the magnetic disk drive 1. In the spindle motor 4 thus constructed, a control current is supplied to the coil 43, whereby the hub 41, the magnet 45 and the back yoke 46 are rotated at high speed together with the magnetic disk 2.
In recent years, it is demanded that data be recorded at high density on a smaller magnetic disk 2 so that the magnetic disk drive may become smaller. Accordingly, pitches in order of microns have been employed for data track pitches.
In order to provide such narrow track pitches in the magnetic disk drier 1, however, NRRO (Non Repetitive Run Out) of the spindle motor 4 must be reduced in the mechanical system of the magnetic disk drive, and the rigidity of the whole mechanical system including the carriage 5 must be increased. The magnetic head must be accurately positioned at a predetermined position in the magnetic disk drive. To this end, accurate servo data needs to be written in the magnetic disk.
In recent years, it is also demanded that the storage capacity of a magnetic disk drive be increased. The number of magnetic disks mounted in the magnetic disk drive is directly proportional the storage capacity of the drive. Thus, to meet the demand for increasing capacity, it is necessary to mount as many magnetic disks as possible within the limited space available in the casing of the magnetic disk drive.
As described above, in the conventional magnetic disk drive, a maximum number of magnetic disks need to be mounted within the casing to meet the request for a greater storage capacity. To provide a space accommodating the disks, the base of the casing should be thin. If the base is this, however, it cannot withstand the load, i.e., the spindle motor and the associated components. The thin base for the casing would reduce the rigidity of the mechanical system. This would increase the possibility of a resonance.
The resonance may cause inconveniences like those described hereinbelow. Servo data would be written in a vibratory manner during servo writing, making unstable the servo data referenced as a yardstick in positioning. Consequently, it would be impossible to position accurately.
If servo data has been written accurately and the resonance occur in use of the magnetic disk drive, it is impossible to keep the accuracy of positioning high level. In any case, if the resonance occur, it is impossible to keep positioning accuracy high.
FIG. 3 illustrates the deformation of the spindle motor 4 because of the occurrence of the resonance. The bearing 42 rotatably supporting the shaft 41a of the hub 41 is shown in its deformation. As shown in FIG. 3, when the magnetic disk 2 inclines and falls down, the magnetic disk 2 is displaced in its radial direction against the magnetic head 3. Such displacement of the magnetic disk 2 in its radial direction causes a positioning error and makes it impossible to position accurately. The deformation of the bearing 42 is shown in FIG. 3. However, because of the occurrence of the resonance, the motor base 48, the magnetic head 3 or the carriage 5 may be deformed. In this case, as in the case of that shown in FIG. 3, it is impossible to position accurately. The positioning error means the quantity of head positional shift from the position in which reference servo data has been written, and thus this head positional shift will be called a positioning error hereinafter.
Conventionally, writing of servo data in the magnetic disk, that is, servo writing has been performed at writing rotation frequency (rotation speed) equal to disk rotation frequency (disk rotation speed) during use of the magnetic disk drive. In recent years, methods described hereinbelow have been employed.
In the first method, the quality of servo data written in the magnetic disk is made stable by performing servo writing at such a rotation frequency that resonance is hardly occur.
In the second method, the quality of servo data is made stable by performing servo writing at a rotation frequency much lower than that of the magnetic disk used in the magnetic disk drive.
In the case of the first method, even if a rotation frequency is determined so as to make it difficult for the resonance to occur, there is variance in resonance frequencies depending on the magnetic disk drive itself, for instance, variance in accuracy or assembling of parts constituting the magnetic disk drive. Therefore, it is difficult to completely prevent the occurrence of the resonance.
In the case of the second method, servo writing is performed at a rotation frequency considerably lower than that in use, and vibrations are reduced by minimizing vibratory energy. However, this also entails some inconveniences.
When a rotation frequency is determined to low level, the floating height of the magnetic head from the magnetic disk declines and thus the head may come into contact with the disk. Therefore, even if the rotation frequency is determined to low level, it is also difficult to completely prevent the occurrence of the resonance.
A condition for using the magnetic disk drive when servo writing is to be performed, is usually different from that in using the disk drive. More particularly, since the servo writer has been connected to the magnetic disk drive, the state of the magnetic disk drive, during servo writing, is different from that when this disk drive is used singularly. Therefore, even if a rotation frequency is determined so as to prevent the occurrence of the resonance and when the magnetic disk drive is singularly used, the resonance may occur during servo writing.